The use of combinations of organic compounds containing at least two intramolecular carboxylic acid anhydride groups per molecule, in particular copolymers containing succinic acid anhydride groups and polyhydroxyl compounds as binders for lacquers and coating compounds has already been disclosed in EP-A-48 128. The application of the principle of this prior publication to the reaction of amines with anhydrides, however, encounters the difficulty that the reaction of amines with anhydrides proceeds very rapidly even at room temperature with the elimination of anhydride and the formation of cross-linked products. The consequently extremely short working life of such combinations have therefore hitherto prevented the use of combinations of polyanhydrides and polyamines in coating systems.
The possibility of a solution to this problem is indicated in DE-OS 2 853 477 which describes mixtures of polyanhydrides and blocked polyamines which have a sufficiently long pot life but harden to form cross-linked products when water is added. Ketimines and aldimines obtained by the reaction of polyamines with ketones or aldehydes are described as suitable blocked polyamines.
As polyanhydrides there are mentioned compounds containing at least two cyclic carboxylic acid anhydride groups per molecule, in particular reaction products of polyols with cyclic dianhydrides in a ratio of hydroxyl groups to anhydride groups of 1:2 or copolymers of an unsaturated cyclic anhydride with polyunsaturated compounds or .alpha.-olefines.
The olefinically unsaturated compounds to be used for the preparation of the copolymers are treated only very briefly in the general description of DE-OS 2 853 477 and in particular there is no indication of the proportions in which the individual monomers are to be used for the preparation of the copolymers. The specific disclosure in the examples of practical embodiments is limited to copolymers of butadiene oil and maleic acid anhydride in proportions of 1:1 and to copolymers of wood oil and maleic acid anhydride. These copolymers have, however, certain disadvantages in that their combinations with bis-ketimines or bis-aldimines would lead to strongly discoloured products. Moreover, the coatings obtained from coating compounds containing unsaturated oils such as butadiene oil or wood oil as their binder components would readily become brittle and would not be weather resistant.
As disclosed in the examples of practical embodiment in DE-OS 2 853 477, the binders specifically described there require the use of dimethylformamide as solvent for working them up and the solids contents, which are only about 20%, are unacceptable.
Another possibility of increasing the pot life lies in the use of oxazolanes instead of amines.
Water hardenable compositions of oxazolanes and polyanhydrides are already known in principle from DE-OS 2 610 406, according to which oxazolanes are combined with polyanhydrides for the preparation of water hardenable sealing compounds and adhesives.
Reaction products of polyunsaturated fatty acids with maleic acid anhydride and polyanhydrides of C.sub.3 -C.sub.6 -alkyl(meth)acrylate and maleic acid anhydride, in particular butyl acrylate and maleic acid anhydride, are described as suitable polyanhydrides.
Experiments carried out by the present Applicants have shown that the systems specifically described in DE-OS 2 610 406 leave much room for improvement with regard to their suitability for the preparation of high quality, colourless lacquer films of great hardness and resistance to solvents and chemicals. This applies both to the systems based on copolymers of maleic acid anhydride and butyl acrylate described in the Examples and to the systems based on reaction products of maleic acid anhydride and polyunsaturated fatty acid esters, which result in end products which tend to yellow.
It was therefore an object of the present invention to provide new binder combinations based on polyanhydrides and modified polyamines which would be suitable for the preparation of high quality systems which could be cured under the influence of atmospheric moisture, which binder systems would have a sufficiently long pot life and would harden within acceptable lengths of time to form clear films resistant to yellowing and to solvents.